Cathode-directed Streamer Research Articles

Ionizing Waves in Surface Dielectric Barrier Discharges Plasma Actuators

Plasma actuators have been proposed as a tool to produce hydrodynamical effects in the boundary layer of aerodynamical flows. We have analyzed some properties of these systems using suitable plasma diagnostics based on the emissivity characteristics of such plasmas. The direction and the velocity of propagation of the ionizing wave spreading on the dielectric surface were measured (in the 100–200 km/s range), and it was demonstrated that it behaves like a cathode-directed streamer. The averaged electron temperature (4–5 eV) and the reduced field strength (E/N ≈ 6 × 1019 V·m2) of the ionizing wave switching the discharges on were measured, too.

Streamer-induced kinetics of excited states in pure N2: I. Propagation velocity, and vibrational distributions of N2(C) and N(B) states

The emission spectra of a streamer discharge in pure nitrogen provide an important tool for investigating the fundamental kinetics of excited electronic states of N2 and benchmark data for validating advanced kinetic schemes for numerical models. In this work, we characterize a streamer monofilament developed in a dielectric barrier discharge configuration, including electrical characteristics, time-resolved images and N2/N emission spectra, all acquired with nanosecond temporal resolution. Time-resolved images and emission characteristics provide clear evidence of the formation of a cathode-directed streamer and allow determining the streamer propagation velocity and the typical values using the intensity ratio of nitrogen spectral bands in the center of the discharge gap. We also measure the vibrational distributions of the N2(C, 0–4) and N(B, 0–2) states. The population of N2(C, 0–4) state, initially formed by energetic electrons in the streamer head, changes later significantly due to the decrease in the mean energy and concentration of the streamer channel electrons. After a few tens of nanoseconds, the electron-impact excitation rate of N2(C) becomes negligible compared to its population by the N2() + N2() pooling. The experimental findings are supported and consistent with the 0D state-to-state kinetic model results and reveal the participation of high vibrational levels of N2() in the pooling reactions.

Numerical modeling of different streamer breakdown modes of gas switch under the external circuit

Gas spark switches had been applied extensively in the field of pulse power technology on account of the advantages of a simple structure, short breakdown time, and high through-current capability. This paper investigates the streamer breakdown process in a two-electrode switch in a nitrogen environment under atmospheric pressure using a numerical method. First, a two-dimensional fluid simulation was carried out for the streamer generated under the negative and positive trigger voltages, and the difference in the breakdown mechanism in the two modes was analyzed. The study showed that when a negative trigger voltage is applied to the rod electrode, a negative streamer from the cathode to the anode will first be formed, and then a positive streamer from the anode to the cathode will be formed. When a positive trigger voltage is applied to the rod electrode, the streamer propagation mechanism is significantly different from that obtained when a negative voltage is applied. In this case, the breakdown mode is a cathode-directed streamer. Finally, we investigated the effect of circuit parameters on the breakdown process. The time scale of breakdown varies greatly with the capacitance. The significant difference in the breakdown process may come from the charging process of the capacitor. The larger the capacitance is, the slower the charging process will be.

On Calculating the Phase-to-Phase CFO of Overhead Transmission Lines Based on the Spatial Minimum Electric Field

A spatial approach for calculating phase-to-phase switching impulse critical flashover voltage of overhead transmission lines is implemented using the Fourier enhanced charge simulation method (FEnCSM). Cathode-directed leader and streamer characteristics are used in an engineering model of pre-breakdown conditions. Rather than identifying breakdown based on complete bridging of the air gap, the proposed method identifies breakdown based on the ability of the leader-streamer system to survive the region of minimum electric field between phase conductors. Use of the minimum field as a basis for identifying breakdown allows anode-directed negative streamers to be reasonably neglected. Combined with FEnCSM, the proposed approach enables explicit representation of phase bundle subconductors and avoids the electrode-specific geometric constants or gap factors common in existing methods. Results obtained with the method compare well to four independent sets of experimental data for phase-to-phase flashover from the literature. The method was developed to facilitate ongoing research into practical use of the electric field as a basis for unifying electrical studies in the design of overhead transmission lines, including HSIL and other novel configurations.

Wave-like emission propagation and fine structures at the contact points of adjacent dielectric pellets in packed bed plasma reactors (PBPRs) operated in helium

Packed bed plasma reactors (PBPRs) inherently have complex geometries where the volume between the electrodes is filled with dielectric/catalytic pellets to form a large array of voids. While the dimension of the plasma region can be several centimeters, the size of a single void at the edges and pores of dielectrics/pellets can reach micrometer dimensions. The understanding of plasma propagation on these diverse length scales is essential for optimizing and controlling plasma processes performed in such discharges. It is known that plasmas are generated in PBPRs as multiple pulses due to cathode-directed positive streamers in the volume, surface ionization waves, or surface streamers over the dielectric surface and stationary microdischarges at the contact points of adjacent dielectrics. In this work, we have investigated the discharge formation and propagation as a function of applied voltage in simplified PBPRs with a single layer of hexagonally arranged hemispherical pellets, operated in helium, using phase and space resolved optical emission spectroscopy. Despite similar discharge conditions at multiple positions, the emission intensity during each pulse spreads like a wave from the center to the edges in the whole discharge cell. The emission due to surface ionization waves is significantly reduced compared to earlier works. These observations could be explained by possible interactions between adjacent microdischarges, already known in other arrays of microdischarges or adjacent streamers. Higher resolution images of the contact points show that the discharge has fine structures with stronger emission at the edges of the contact points; this effect is enhanced as a function of the driving voltage amplitude. This is possibly the consequence of non-uniform electric field distribution at the contact points due to the polarization of dielectrics.

Dynamics of negative coronas in airflow

Recent numerical studies captured the effects of airflow on positive streamers, when subjected to a lateral wind. The positive streamers initiate from the trailing side of the anode and propagate with a clear tilting in the direction of the airflow. In this paper, we present the results of a simulation of two successive negative corona pulses and investigate their behaviour when exposed to a lateral wind. We use a three-dimensional numerical model that couples the electrical discharge dynamics with the airflow and allows the study of the discharge on electrons and ions timescales. The results show that negative coronas are not easily influenced by the airflow and tilt just slightly in the flow direction. Our simulation explains the physical mechanism that governs the formation and termination of negative corona pulses (Trichel pulses) and describes for the first time the proposed cathode-directed streamer mechanism for negative corona current pulses.

Imaging and simulations of positive surface and airborne streamers adjacent to dielectric material

This paper presents the imaging and simulations of positive airborne and surface streamers in the air that is adjacent to a dielectric material. The goal of this paper is to investigate the intriguing observation of the occurrence of airborne streamers that originate at a dielectric surface at a distance from the tip of a high-voltage electrode. This effect may be attributed to the accumulation of charges at a spot on the dielectric material during subsequent streamers. Such a positively charged spot causes airborne streamer inception after reaching the critical value of the charge density. This effect is observed in a side-view configuration by high-speed imaging in a time-accumulated and spatially resolved mode. The hypothesis of the occurrence of airborne streamers in certain sequences was complemented by 2D simulations. The surface charge densities were quantitatively evaluated by the simulations. Two types of streamers were simulated: first – without an accumulated surface charge; and second – with a spot of a defined charge density (which triggers the airborne streamer). After reaching the critical value of the charge density, such spots that are charged during subsequent cathode-directed streamer events may trigger a discharge towards the air side. The length of the surface streamer along the dielectric surface is voltage-dependent. The experimental and simulation results extend the insight to streamer mechanism in configurations with charges accumulated on the dielectric surface – including the inception of an airborne streamer.

Generation of excited species in a streamer discharge

At or near atmospheric pressure, most transient discharges, particularly in molecular gases or gas mixture containing molecular gases, result in a space charge dominated transport called a streamer discharge. The excited species generation in such discharges forms the basis for plasma chemistry in most technological applications. In this paper, we simulate the propagation of streamers in atmospheric pressure N2 to understand the energy partitioning in the formation of various excited species and compare the results to a uniform Townsend discharge. The model is fully two-dimensional with azimuthal symmetry. The results show a significantly larger fraction of the energy goes into vibrational excitation of the N2 ground state in a streamer-type discharge in comparison to a Townsend discharge. For lower applied voltages, the anode-directed (negative) steamer is slightly more efficient in channeling energy for excited species production in comparison to a cathode-directed (positive) streamer. Near 70% overvoltage, both types of streamers show very similar energy partitioning, but quite different from a Townsend discharge.

Theoretical and numerical study on breakdown mechanism of nitrogen spark switch

Compared with the two-electrode gas spark switch, the three-electrode gas spark switch has high controllability, low working voltage and small jitter, so the three-electrode gas spark switch is widely used in pulse power technology. The discharge of gas spark switch is high pressure gas discharge, which is characterized by high electron collision frequency (10<sup>12</sup> Hz), small mean free path (10<sup>–6</sup> m), short breakdown time (10<sup>–9</sup> s), and complex physical process (including the secondary electron emission, the generation of seed electrons, the space charge effect and various collision processes between electrons and nitrogen molecules, etc). At present, it is difficult to quantitatively describe the breakdown process of the three-electrode gas switch, and the detailed theoretical research is lacking. Therefore, the breakdown mechanism of atmospheric pressure nitrogen spark switch, including two-electrode and three-electrode, is studied theoretically and numerically in this paper. The purpose of this study is to compare the simulation results of the two different gas spark switches, and obtain the characteristics of stream breakdown in different gas spark switches. Firstly, the numerical simulation and theoretical analysis of two-electrode gas spark switch are carried out. According to theoretical and numerical calculation, it can be found that for the plate-plate two-electrode switch, the stream breakdown cannot be generated under low voltage (less than 6.3 kV), while under high voltage (more than 6.3 kV), first the anode-directed streamer is formed, and then the cathode-directed streamer is created. In addition, the simulation results show that the plasma generated by the trigger can effectively reduce the breakdown voltage. Finally, the three-electrode gas spark switch is studied theoretically and numerically. It can be seen that in the breakdown process of the three-electrode gas spark switch, the breakdown first occurs between the trigger and the insulator, and then this plasma channel expands to the anode and cathode, finally forming the arc channel between the anode and the cathode. Under the calculation conditions in this paper, if the cathode-trigger and the anode-trigger are required to break down simultaneously, the applied voltage between the cathode-trigger should be greater than 1.18 kV, while the applied voltage between the anode-trigger should be greater than 3 KV. When the field emission of the trigger is considered, the breakdown threshold can be significantly reduced.

Relativistic runaway electron avalanche

Discussed are the genesis of the concept of the relativistic runaway electron avalanche (RREA) and its mechanism as an analog of the Townsend’s avalanche, but capable of developing, unlike the latter, in weak thundercloud electric fields. Thanks to this, it was possible to overcome difficulties while interpreting results of observations of penetrating emission enhancements in thunderstorm atmospheres. The main inelastic interactions of high-energy electrons with atomic particles participating in the avalanche development are described; in terms of the drag forces, the essence of the runaway process is discussed; and methods of RREA numerical simulation are described. In approximate historical sequence, results of calculations of the spatial and temporal scales of the avalanche enhancement are analyzed and contemporary data on avalanche macroscopic characteristics are given, which is required for numerical simulations of the runaway electrons in the fluid approximation. As an extension to the relativistic range of the mechanism of the classical cathode-directed streamer, relativistic positive feedback is discussed, by means of which a generation of the RREA series, as a self-sustained process, is supported. Laboratory experiments on RREA modeling are described, in one of which the initial stage of the avalanche was produced. The historical background of the issue is often more interesting than the results obtained. I P Pavlov (1849–1936), 1904 Nobel Prize winner in Physiology and Medicine

A New Computer Simulation of Avalanche Evolution and Its Transition Into Anode- and Cathode-Directed Streamers in Short Uniform Field Gaps

A new method based on the charge simulation technique is proposed for computing the electric field in a short uniform field gap with space charge and without space charge of avalanches growing in the gap. The self-space-charge field of the avalanche is evaluated to result in field enhancement ahead of the avalanche due to electrons at its head and behind the avalanche due to positive ions in its wake. Also, a criterion is proposed for calculating the breakdown voltage and time-to-breakdown when the primary avalanche crosses the gap. The criterion is based on self-recurrence of the avalanche by secondary processes, and the most dominant one is the photoemission at the cathode. The condition of transition of the primary avalanche to anode- and cathode-directed streamers is formulated where the avalanche is chocked by its self-space-charge field. The number of electrons starting avalanche chains forming the streamers is evaluated in the gas ahead and behind the primary avalanche by photoionization and at the cathode by photoemitted electrons. The present computed values of the breakdown voltage of air at different pressures and N2 at atmospheric pressure as well as the time-to-breakdown in air at atmospheric pressure agree satisfactory with those recorded experimentally. The computed temporal variation of the size of the primary avalanche as influenced by its self-space-charge field agrees with that reported in the literature at different $E/N$ values. The computed current waveforms of the primary avalanche and its successor chains agree reasonably with those measured and those calculated using a finite-element method.

Generation of a large-scale uniform plasma plume through the interactions between a pair of atmospheric pressure argon plasma jets

In this Letter, a uniform air plasma plume with a fairly large scale is generated downstream of an air flow through interactions between a pair of argon plasma jets excited by a direct current power supply. With increasing power voltage, two separated plumes are first generated downstream of the pair of jets, which transit to the uniform planar plume with a fairly large scale. The planar plume elongates with the increasing air flow rate or power voltage. Moreover, the temperature of the planar plume is comparatively low with a maximum of about 80 °C, which is symmetrically distributed perpendicular to the pair of jets. For the separated plumes, discharge current is intermittently pulsed under a time-invariant gap voltage. However, both gap voltage and discharge current are periodically pulsed for the planar plume. High-speed video images reveal that the planar plasma plume stems from two discrete plumes flushing out of the pair of jets, which are soon merged together by a cathode-directed streamer due to interactions between them, leading to the generation of an arc-shaped filament. Due to the air flow, the arc-shaped filament keeps moving away from the jet vicinity until it quenches at last, which belongs to a glow discharge regime. In addition, surface modification of polyethylene terephthalate (PET) is performed with our planar plasma plume, which reveals that the hydrophilicity of the PET surface can be improved greatly.

Discharge propagation on a dielectric surface in a single-filament arrangement

Discharge development and streamer propagation on a dielectric surface were investigated in nitrogen-oxygen gas mixtures at atmospheric pressure. A coaxial pin-to-pin arrangement was used to generate single surface discharges driven by positive unipolar square wave high voltages of between 7 and 9 kV at 4.3 kHz. The development of surface discharges was recorded by ICCD and streak cameras. The discharges developed on the surface using pin electrodes attached directly to the dielectric plate. The accumulation over several discharge events showed a propagation front evolving from the electrode tip, while images of single discharges showed a non-uniform and branched structure of discharge channels. The electrode polarity influenced the discharge expansion and propagation velocity. Positive polarity of the metallic electrode (rising slope of HV pulses) led to a cathode-directed streamer with higher propagation velocities (5 × 105 m/s) than for negative polarity (relative to surface charges; falling slope). The increase in the O2 content in N2 from 0.1 to 20 vol% resulted in a decrease in discharge duration and an increase in streamer velocities.

Streamer Breakdown with Runaway Electrons Forming Diffuse Discharges in an Inhomogeneous Electric Field

The results of experimental investigations of formation of diffuse nanosecond discharges in a nonuniform electric field in air and other gases at the pressures within 12.5–400 kPa are presented. The experiments were performed using a four-channel ICCD-camera, an ultra-fast streak-camera, and wideband oscilloscopes. It is found out that spherical streamers are formed in a sharply nonuniform electrical fields under high voltages in a pre-breakdown stage of the discharge, irrespective of the gas used (air, nitrogen, hydrogen, methane, neon, or helium). The data are obtained on the instantaneous streamer velocity in air at different voltages and on the displacement current caused by the electric field re-distribution in the gap during the streamer formation. Beams of runaway electrons are recorded. The mechanism of formation of anode- and cathode-directed streamers at high voltages is discussed.

A Regularly Swelling Hollow Plume Generated in an Atmospheric Pressure Argon Plasma Jet Excited by a Positively Biased Sinusoidal Voltage

Far different from common plasma plumes, a regularly swelling hollow plume is observed in an argon plasma jet. The results indicate that the regularly swelling plume appears only at a low-frequency voltage (lower than 1 kHz) that is positively biased. Waveforms of applied voltage and light emissions reveal that there is a barrier discharge inside the nozzle, which initiates once every half voltage cycle. Outside the nozzle, it initiates once every positive half voltage cycle and never at the negative half cycle. By fast photography, it can be found that the positive discharge stems from the rod end of the powered electrode and bends down to hug the tube surface, which finally propagates to the outside as a cathode-directed streamer due to the charge overflow effect. The bifurcating streamer propagates along the periphery of the argon flow, which results in a hollow structure of the plasma plume. Due to abundant active species provided by the positive discharges inside the nozzle, the diameter of the plasma channel is comparatively larger in the swelling regions than in other regions, which induces the swells of the plume. Furthermore, the negative barrier discharge is diffuse, whose influence on the swells can be neglected. Finally, the contribution of active species generated upstream of the swells is verified by investigating the distance between two adjacent swells as a function of gas flow rate and driving frequency.

Enhancement of atmospheric plasma jet–target interaction with an external ring electrode

Plasma devices generating a sequence of streamers over an inert gas jet in ambient air have been widely used recently in anticancer therapy. An enhancement of electric field carried by the streamers to the target surface is a way to intensify the plasma jet impact on a cell life cycle. In this paper, an increase of the surface electric field without a rise of discharge power is provided with an external biased electrode installed between the plasma device and target. The effect of the presence of this biased electrode on cathode-directed streamer dynamics and its interaction with conductive or charged dielectric surfaces are studied in 2D fluid model simulations and in the experiment. Two stages of the ionization process induced by the streamer near the plasma–target interface are observed in the experiment and simulations. The first stage is related to the streamer approaching the surface. The second stage is the stationary self-sustained ionization and lasts much longer during a positive cycle of sinusoidal voltage applied to the electrode inside of the plasma device. A much higher surface electric field is registered on the dielectric surface compared to the conductive one, due to an accumulation of the surface charge, especially with the negative bias on the external electrode. An area on the dielectric surface exposed by the plasma jet is much larger compared to the conductive one, due to spreading the surface ionization wave over the dielectric.

Investigation of the transition from streamer to uniform ‘overvoltage’ mode of atmospheric air nanosecond-pulsed dielectric barrier discharge

A nanosecond-pulsed dielectric barrier discharge ignited in atmospheric air was studied by optical emission spectroscopy to investigate the time- and space-resolved development of the reduced electric field. The discharge operated in two distinct modes that correlate with the values of the applied electric field: non-uniform streamer regime and uniform breakdown regime. It is shown that the reduced electric field in the non-uniform mode corresponds to the propagation of cathode-directed streamers, while in uniform discharge both molecular nitrogen emission and the reduced electric field are uniformly distributed along the gap during the initial discharge development stage with corresponding lower reduced electric field values.

Plasma Jet Interaction with Dielectric Surface

Propagation of cathode-directed streamer over the helium jet in ambient air is studied in the framework of fluid model and in experiments. The streamer is initiated by applying sinusoidal voltage to the powered electrode inside of dielectric tube and propagates to the direction of treated surface. The gas mixture is calculated with ANSYS Fluent software for helium supply of 4 sl/min. The experimental data on the speed of streamer are in good agreement with calculation results. The reflection of ionization wave from the surface was observed in the experiment and in simulations. It is shown that the reflected ionization wave follows a channel produced by the streamer.

In-situ optical observation of dynamic breakdown process across microgaps at atmospheric pressure

The study on breakdown characteristics at microscale is very vital to the plasma physics community and micro/nanoelectronic industries. However, the dynamic development processes including the electron avalanche and discharge channel during the breakdown event are still unclear experimentally so far due to the restriction of the in-situ diagnostic technique. In this paper, an optical diagnostic technique is put forward to achieve an in-situ observation of the dynamic breakdown process across microgaps at atmospheric pressure. By using the nanosecond temporally and micron spatially resolved optical measurement method, the light emission appearances of microgap breakdown under nanosecond pulse have been captured and imaged directly. The initial and propagation mechanisms for electrical breakdown across the very small air gap are discussed based on the light emission properties. Results show that the light emission initiates from the cathode as the initial electron avalanche, then the avalanche transits to the streamer due to the space charge effect and photoionization, and propagates towards the anode with a velocity of 4.40×10 3 m/s, demonstrating a cathode-directed streamer in the pulsed breakdown. The space charge induced field enhancement and photoionization are proved to play a significant role in the pulsed breakdown across microgaps. In addition, the time to breakdown is estimated to be about 20ns while the formative time lag is about 9ns through the temporal sequence of the optical images. The in-situ optical measurement technique presented in this paper would be of a great help for better understanding the physical mechanism of microgap breakdown.

Negative surface streamers propagating on TiO2 and γ-Al2O3-supported Ag catalysts: ICCD imaging and modeling study

Surface streamers propagating on the surface of titanium dioxide (TiO2) and alumina (γ-Al2O3) were studied in negative polarity using intensified charge coupled device (ICCD) imaging and numerical simulation. Detailed time-resolved ICCD images of cathode-directed streamers (CDSs) emanating from a ground electrode are first presented in this report. Instead of primary streamers in positive polarity, only a glow-like discharge appeared in the early stage at the cathode under negative polarity. After this discharge disappeared, a counter-propagating CDS initiated from the ground electrode (anode). Numerical simulation indicated that strong electric fields at the pellet-anode and the formation of positive ion rich local spots were the main reason for the CDS formation near the ground electrode. The maximum velocity was 750 km s−1 for Ag-supported γ-Al2O3 and 550 km s−1 for Ag-supported TiO2, respectively. In contrast to the CDS in the gas-phase with a positive polarity, the CDS in a catalyst packed-bed under negative polarity showed more branching and a larger number of streamers in the presence of oxygen than in pure N2.